[RESOLVED] Date increment by month: seemingly odd results from strtotime

[phpSimon]

Hi everyone

When looping and incrementing dates, I noticed that one date was incrementing incorrectly. On checking the code I realised I'd done it a rather poor way and fixed it, but does
anyone know why I get the results shown?

$monthnumber = 1;
$from = date('Y-m-01');
$to = date('Y-m-t');

while ($monthnumber < 4) {
        
    print ("Monthnumber $monthnumber From $from to $to<br />");
        
$from = strtotime($from);
$from = date("Y-m-01", strtotime("+1 month", $from));

// returns an incorrect $to date for February
// $to = strtotime($to);
// $to = date("Y-m-t", strtotime("+1 month", $to)); 

// this works properly (and is obviously neater)
$to = date("Y-m-t", strtotime($from));

$monthnumber++;
} // done all months 

Correct output from code above:

Monthnumber 1 From 2012-12-01 to 2012-12-31
Monthnumber 2 From 2013-01-01 to 2013-01-31
Monthnumber 3 From 2013-02-01 to 2013-02-28

Incorrect output from code if I revert to the commented out $to code:

Monthnumber 1 From 2012-12-01 to 2012-12-31
Monthnumber 2 From 2013-01-01 to 2013-01-31
Monthnumber 3 From 2013-02-01 to 2013-03-31

Thank you

[NogDog]

Maybe you could use the "first day of next month" and/or "last day of next month" strotime() expressions, if that's what you ultimately want?

[Bonesnap]

It's probably because you're using 't' in your date() function instead of something like 'd' or 'j'. I can't say for sure that's the issue but they are different and it's what stuck out to me immediately.

The best way of incrementing dates (or working with dates/times in general) in my opinion is using the DateTime class.

[Weedpacket]

$to starts with December 31; December 31 + 1 month is January 31; January 31 + 1 month is February 31.

February 31? That's three days after February 28. So it's either March 2 or 3 depending on whether or not it's a leap year.

The problems would actually continue past this: March 31 + 1 month is April 31 (i.e. the day after April 30).


(Even if that all looks weird, it is consistent, and mimics how addition gets done ("April 30 + 1; 1, and carry the month...").

[dalecosp]

No comment on the OP's issue, you guys have offered plenty of pointers.

Interesting story, somewhat related ... yesterday I was updating a calendar routine I wrote last month to show "the previous month", and it showed that November 4th occurred twice. Had me scratching my head for a while until I realized that 4 Nov was the day after the US went back to Standard Time from Daylight Savings. I was doing something like "$ts+=86400" to loop through days; when the calendar added that extra hour ... [insert lightbulb moment here] ... problem now solved.

If there's a "moral" to this story, it's "use PHP built-in functions whenever possible"

[bradgrafelman]

I was doing something like "$ts+=86400" to loop through days

Reminds me of my reply here.

[phpSimon]

Thanks everyone for the usual enlightening help!

NogDog, I didn't know about the "first day of next month" and/or "last day of next month" strotime() expressions. I can't find a definitive list of strtotime options but I'm sure there's one somewhere. strtotime is amazing.

Bonesnap, thanks for the DateTime class which I didn't know about either. I do a lot of date manipulation so that'll help a lot.

Weedpacket, great point, I can see the consistency now that you point it out. Of course, strtotime doesn't know that adding one month to a date means I necessarily want the last day of the month; even if the current date 'happens' to be the last month day. I presume it just increments the month number and applies a date format to it; I guess that step increments the year and resets the month to 1 if required.

dalecosp and bradgradfelman demonstrate the point further very well; use PHP built-in functions to work with dates.

Thanks again everyone.